.header{
    display:flex;
    /* 水平垂直居中 */
    justify-content: center;
    align-items: center;
    width:100%;
    height:100%;
    /* background-color: red; */
    /* 背景颜色动画设置 */
    transition: background 0.5s;
}

.header-transition{
    background-color: rgba(25,196,138,0.9);
}